It is well known that there are many applications for which common steel cannot be used unless it has a protective coating, because of its low corrosion resistance. One of the most important of the many methods of protecting such a steel is to coat it by dipping it in a molten bath of metal, particularly of zinc and its alloys.
The corrosion behavior and mechanical properties of products with zinc-based coatings depend largely on the conditions under which the galvanizing is done and the composition of the molten bath. Particularly, bath composition largely governs the growth of the brittle phases at the base/coating interface and the chemical stability of the external phase (known as the h phase) of the coating, which largely controls the corrosion behavior of the galvanized piece.
The most widely used bath consisting essentially of zinc give rise to numerous problems as regards the brittleness of the resulting coatings and the corrosion resistance thereof, particularly in atmospheres where there are chlorides or sulphur-based acids (derived from SO2, H2S, etc.).
Numerous coating alloys have been developed to overcome these difficulties. They can be classed as high, medium and low-percentage alloys.
An example of a high-percentage alloy is one containing about 50% aluminum, described in U.S. Pat. No. 3,393,089 in the name of Bethlehem Steel. The resulting coated product, sold under the trade name "Galvalume," has a very good corrosion resistance.
Medium-percentage alloys include, for instance, a zinc alloy with 3% Mg and 4.4% Al, British Pat. No. 1,125,965 and a Zn alloy with 3% Mg and 1% Al, Belgian Pat. No. 814,696; this latter in the name of Centro Sperimentale Metallurgico Spa., describes an alloy for coating ferrous materials, which is particularly resistant to localized corrosion and polarity reversal in hot water, and is especially suitable for coating pipes.
Then there are zinc-based alloys with a low percentage of alloying elements, e.g., British Pat. No. 1,057,285 in the name of Armco Steel Co. describes one with 0.04-0.35% Al and 0.01-0.1% Mg.
At present, continuous coating lines for sheet and wire, the Sendzimir process is used; this generally utilizes an alloy containing 0.15-0.30% Al and around 0.2% Pb. In recent years, this process has gained steadily in importance because of the increasing tendency to form light and medium-weight sections from flat galvanized steel, which is bent and formed into box sections.
However, the products obtained at present with the Sendzimir process do not have satisfactory corrosion-resistance properties. For instance, they will not withstand attacks in chloride or sulphur-acid atmosphere, nor they are resistant to underfilm corrosion.
Though much study was effected about this problem, so far no one appears to have producted a galvanizing alloy capable of replacing that used industrially in the Sendzimir process, without needing substantial modifications to the plant and process, and which is also capable of improving the corrosion resistance of the products obtained and for broadening their fields of use.